texts: Yet another Question about the expansion of universe

texts:
A galaxy 13 billion light-years distant is 94% of the way to the big bang. The universe was 800 million years old when the light was emitted. That galaxy today is traveling away from us at 94% of the speed of light roughly. So 13 billion years ago it was 750 million light years away from us. Assuming the expansion is NOT greater than light speed, that photon did NOT go backward. This is also evidenced by the fact that the photon arrived at earth. It seems to me the photon took 13 billion years to cross 750 million light years. What am I doing wrong?
   Question two. If this is right then please explain how we go from 750 (or 800) million light years to 46.5 billion light years.


texts: As days get longer, why isn't the added daylight split evenly between sunrise and sunset?

texts:
I have a basic understanding of the sinusoidal relationship of the rate of change of daylight as the year goes on. But it seems intuitive that the changes should be equally split between sunrise and sunset.

For my location, Bend Or., todays sunrise/sunset is: 07:40 and 16:43 And for January 31st it is: 07:24 and 17:14

So sunrise will be 16 minutes earlier and sunset will be 31 minutes later. It appears that there is twice the added daylight to sunset than to sunrise. Why aren't the differences equivalent?


texts: Light beam (1 photon) in the limited universe?

texts:
If our universe is not infinite, what happens with the light beam (or photon) when it will travel through the whole universe?
For example, observable universe according to wikipedia has diameter 93 billion light-years. What will happen with photon after 93 billion years if I generate it now? Will it return back to me? Or it will never reach the "edge" of the universe because the universe is expanding faster?

UPDATE 1

Of course, I assume that photon will no be absorbed :) because it is just a theoretical thinking. And return back I mean not exactly to the same point, but approximately within some range (let say 5% of the universe size).

Do I correctly understand that there are a few options for the case when the universe is finite (means "closed geometry"):

Photon will never return back because the universe will collapse before it (in case if there are no dark energy).
Photon will never return back because universe expanding faster than light speed.
Photon will come back because universe expanding slower than light. And what
of these 3 options is most probable from a current scientists point of view?


texts: For any given meteor shower in a given year (e.g. Quadrantids, now) where can I see a rough prediction of how many days it will last?

texts:
For a given shower the rate hitting the Earth's atmosphere versus time is usually a peak with a width of a few days, though there are some exceptional unicors.

Is there a place where I can view this envelope?

The context here would be to have some idea for a given year of a given shower how much less intense it would be one, two, or three days before or after maximum. I'm not looking for something precise or location dependent, but just to get an idea how critical it is seeing it on the "best" night versus a more opportune night.

Is there some place to go to see some predictions of many of these envelopes in order to look up a given one, or to see which ones are narrow and which ones are wide?


texts: After Pluto's demotion as a planet, we have currently eight planets in our solar system. But Sun's gravitational pull can be felt well beyond Pluto, so is it possible to have a ninth planet beyond Pluto? I asked this question because we have Kuiper belt beyond Pluto, so there might be some Kuiper belt object which may qualified to be a ninth planet, isn't it possible?


texts: Is there a far away probe that has recorded the light spectrum from Earth as if it were an exoplanet?

texts:
We analyze the light spectrum of stars and distant planets to know characteristics like: chemical composition, temperature, mass, etc.

Starting from the same principle, a probe at the border of the solar system could analyze the spectrum of the light reflected by the Earth. In theory, comparing the spectra captured by the probe with what was happening here on Earth at a time, the light was reflected to the space we could discover new spectrographic patterns that would help us look for planets similar to Earth and maybe even life.

I wonder if any study like this has been done, where a whole-Earth spectrum from a distant spacecraft has been captured and analyzed, with the intent of working the problem as if it were a spectrum from an exoplanet, in order to correlate with known Earth conditions.


texts:
Mathematically, for black holes old enough that the stellar material has collapsed all the way into
the singularity, the region between the horizon and the singularity is occupied by a spacetime
where the time and space coordinates are reversed from those of the outside world. What this
means in terms of what you experience is unknown. Other more complex conditions can occur of
the black hole is rotating. In that case the singularity becomes a ring around the center of the
black hole. You can pass through the center, but the tidal gravitational field would be lethal in all
likelihood. In nearly all cases there would be gravitational radiation rattling about, and this would
cause distortions in spacetime that would probably lead to spectacular optical distortions.
My question is Black hole absorbs everything. Is there any mathematical proof regarding this?


texts: I too saw a star like object moving across the sky it appeared to be a star moving!

texts:
I too saw a star like object moving across the sky from Bournemouth towards say Parkstone, it appeared to be a star moving for quite a long time until it disappeared from my view!
I have seen this the last two nights and so have my family, it's not the Space Station as I see this regularly as my Star Walk app, you can add in when you know what the object is! But unless I know the name of this object I can't track it on my app, does anyone know what it could Be?
It is not a plane as far too high like a star moving but fast across the sky? This was timed 22:10 on Sunday August 28th 2017


texts: What's this red stuff in some astronomical photos, e.g. Centaurus A?

texts: Sorry for the rookie question, but I've got no idea about that red "belt" crossing over center of the galaxy, in this photo. I've actually tried all sorts of google searches but I couldn't get anywhere with keywords like "The red stuff" or so. Years ago I had a color book in which I saw this photo for the first time, and I can remember my attempt to remove the red dirt and clean the image with my shirt!


texts: Estimating the tangential and cross component of the galaxy's shear using Gnomonic projection

texts:
I would like to know how I can estimate the tangential and cross component of the galaxy's shear using Gnomonic projection
of the right ascension and declination the galaxies relate to the center of galaxy cluster and estimate?
Edit. Shear is the second order moment of a galaxy's surface brightness. You can find more on the topic in the following wiki on weak lensing.


texts: How small would you have to crush an object for it to become a black hole?

texts: I was wondering how much you had to crush an object for it to become a black hole. Recently I learned that anything could become a black hole (even you) if it were crushed down small enough, for example our sun would have to be crushed to the size of Manhattan to become a black hole (According to a video I saw). I did a little research and found out that something would have to be crushed down to Schwarzchild radius meaning all of that objects mass would have to be crushed into that objects radius. But if you say crush the sun down to the size of Manhattan, that is much smaller than the sun's actual radius. So I haven't really gotten a clear answer and I'm a little confused, thanks for clarifying =)


texts: Is it possible for two stars to be in a horseshoe orbit around a much larger star?

texts:
I was reading about how Saturn has two moons, Janus and Epimetheus, that swap orbits once every four years. Could something like this happen on a much larger scale, but with stars instead of a planet and moons? Let's say you had a large, massive star with two red dwarfs orbiting around it. Could those two red dwarf stars fall into a horseshoe orbit, regularly swapping orbits every few years or so?

more about horseshoe orbits


texts:
I'm a third year undergrad, and for the project I'm doing this summer, I need to figure out the Earth's position and velocity from the moon's point of view. From the reading I've done, it seems like there are formulae to convert from the orbital elements of the earth-moon system to a state vector describing the moon's position and velocity in the earth's reference frame. So, I'm wondering if because of the symmetry of the problem (i.e. from the moon's POV, the earth orbits it in an ellipse), there is a way to convert the easy-to-find orbital elements of the earth-moon system to the orbital elements of the earth as seen from the moon? I could then just do the conversion to the state vectors that I'm looking for.

Any sources/help is much appreciated!


texts: Why stars are shown as crosses in some Hubble images?

texts:
Why the most luminous stars (point sources of light) like the ones in the image below are rendered as crosses? If they are artifacts generated by some Hubble instrument, what causes them? Can them be corrected in some way?

What does the orientation of the crosses depend on?

(image source)


texts: Does the sun cross other spiral arms in its movement around the galaxy's center?

texts:
Today, Ansa.it released an article that states:

  [...]. In questo suo peregrinare galattico, il Sole ha attraversato anche
  i due bracci della Via Lattea Perseo e Centauro. "Sono zone di alta
  densità stellare, in corrispondenza delle quali il Sole e le stelle
  intorno rallentano e possono anche fermarsi, [...]

(my) translation to English:

  In this galactic wandering, the sun traversed the two spiral arms of the Milky Way, Perseus and Centaurus.
  These are regions of high density of stars, in correspondence of which
  the Sun and other surrounding stars slow down and can even stop, [...]

The article is complemented with a suggestive video-clip that depicts the Sun orbiting around the galaxy's center and crossing various spiral arms.

Given my lack of an education on the topic, I cannot reconcile the existence of an entity called "spiral arm" with the notion that a star can freely cross several "spiral arms" in its movement around the galaxy's center.

My intuition is that if every star within the Milky Way could freely cross the "spiral arms" several times during their life-span, then there would be no such thing as a "spiral arm" at all (because this is supposed to be a mass grouping wherein all matter moves --please forgive me the abuse of the word-- "together").

Where am I wrong?
Is the above description of the movement of the Sun accurate?
In case of an affirmative answer, is it a very special case or is it a defining characteristic of the entire Orion's arm?
In the latter case, can other spiral arms cross each other?


texts:
I'm curious if there is a readily accessible tool available, or simply a reliable way to find out what the distance of the planets in the solar system were on a given date.

For example, if I am trying to determine how far:

The Sun
Mercury
Venus
Mars

were from Earth on May 3, 1954 ... how would I do that?


texts: Did simultaneous transits of Mercury and Venus actually last occur in 373,173 BC?

texts:
According to the Wikipedia article for Transit of Venus, the last time a transit of Mercury and a transit of Venus occurred at the same time was 22 September of 373,173 BC. This is a Featured Article and the statement has been on the page since December 2008, implying authenticity. But I cannot find an academic citation anywhere to back this up.
I can find an academic citation for the occurrence of the next one, in 69,163 AD. But I cannot confirm the previous one.


texts: I am an amateur astronomer currently working on Spitzer telescope dataset with python (astropy and aplpy). From the examples in astropy's website, I understood that the FITS data is a way to represent image data with human-readable headers.  However, I now have a fits image ( SPITZER_I2_46469632_0000_0000_2_bcd.fits ) which is a 3D array of dimensions (64, 32, 32). I believe the second and third coordinates to be the length and height of the image in pixels. What about the first dimension? How do I interpret such images and NaNs in the data?


texts: I have two short telescopes. The tubes that the eyepieces fit in are different; one is short and one is long, such as the tubes on the Celestron first scope and the Jason Comet chaser. Why is this so and what is the advantages of one over the other? They both take 1.25 eyepieces.


texts: Finding the radius of an eccentric orbit at any point

texts:
Knowing the apoapsis, periapsis and therefore period of an orbit, how can I find the radius of an object in an orbit at a given angle or time, whichever is needed.

For example, if I have an object m orbiting around object M with an apoapsis of 100m and a periapsis of 50m, what is the distance between m and M after t seconds, or at $\theta$ degrees. Which ever is needed. I say 'or' because I don't know what you would use to find the radius.


texts: Are there viable explanations for the red shift of distant galaxies other than the accelerated expansion of the universe?

texts:
I was told that photons lose energy (= increase their wave length) when moving away from mass and gain energy moving towards mass.
Could this increased red shift of photons, originated a long time ago, come from the fact that they started in an old, dense universe and end up in a young, less dense universe?


texts: What does the 'amplification factor' of gravitational microlensing mean?

texts:
At first I assumed the amplification factor meant an increase in brightness. But since the equation: $$A(u)=\frac{u^2+2}{u\sqrt{u^2+4}}$$ is independent of mass, this make no sense to me. Wouldn't everything be infinity magnified (if its mass independent empty space should work, right)? Even if its simply not valid for a zero mass, everything would be amplified by an arbitrarily high amount any time even a neutrino passes arbitrarily close to the right $u$?
But if its not brightness, I don't know what signal is being amplified.


texts:
My brother noticed something very odd last night: it was amazingly light out when he stepped outside for a smoke even though it was between 3 AM and 3:30 AM EDST and we are near Toronto Ontario Canada, not in the Far North. He is outside at that time most nights and is no stranger to being up very very early at all times of the year. He has always lived here and is 63 years old, not an inexperienced child. He said the amount of light in the sky was equivalent to the amount of light you'd see in the mid-afternoon of an overcast day. He'd never seen anything like it.
Since I have a lifelong love of science fiction, he asked me if I had any idea why this might have been; he thought there might be some natural phenomenon that might explain it. I asked if he had used some kind of intoxicant but he assured me he was completely sober.
I'm at a loss to think of any plausible source for his experience but I'm not a scientist of either the professional or amateur kind so I thought I'd ask here if there is some rare atmospheric condition that might explain it.


texts: Fate of helium moving toward a singularity

texts: On the molecular level, if an atom, consider helium, were to fall into a black hole, what would be the ultimate fate of its electrons, protons and neutrons?


texts: The light of which star is next to reach the earth?

texts:
I heard that the sky wouldn't be dark at night if the light of every star had already reached the earth.

Of which (currently invisible) star will the light reach the earth next and when will that be?
What was the last star that we can now see at night to reach the earth and when was that?
How can we know (1) if the light hasn't even reached us yet? Or can't we know and will a star suddenly appear without anyone being able to predict that?


texts: Is is possible for a main sequence star to have 0.77 solar masses yet only be 0.54 solar radii?

texts:
I know ordinarily a star's mass and radius tend to be proportional, so a 0.5 solar masses star tends to be about 0.5 solar radii and have a density of 4.82 g/cm³, 3.42 or about three and a half times the Sun's.
But, is it theoretically possible to have a main sequence star with 0.77 solar masses while still only being 0.54 solar radii?  This comes out to a density of 6.88 g/cm³, 4.88 or about five times the Sun's.
This represents a density 1.43 or about one and a half times denser than "usual".
Can this happen in nature?


texts: How to tell between annular and total eclipse (directly)?

texts: An annular eclipse is similar to total eclipse except our Moon "apparently" appears smaller than the Sun, my question is since in both cases there will be a glowing ring forming... so how to tell them apart?


texts: Why does shell fusion produce more energy than core fusion?

texts:
Stars go from the main sequence phase to the red giant branch due to the depletion of hydrogen in the core. As a result, the star contracts and shell hydrogen fusion begins, which apparently produces much more energy. Why does this produce more energy than core hydrogen fusion? The Wikipedia article on red giants says that this is because of higher temperatures, but why does shell hydrogen fusion occur at higher temperatures than core hydrogen fusion?

After the helium flash in low-medium mass stars, does shell hydrogen fusion continue to occur? If so, then why does there a drop in luminosity from the helium flash to the horizontal branch? And if not, then why not?

Similarly to the first question, in the AGB phase, why does shell helium fusion produce so much more energy than core fusion?

Why does the early AGB phase derive most of its energy from shell helium fusion and not shell hydrogen fusion?


texts: Why is this telescope so short? How hard is it to make such a fast primary?

texts:
The recently discovered object C/2019 Q4 (Borisov) is in the news because it might have an eccentricity greater than one, which means it might be from outside the solar system, though not necessarily.
MPEC 2019-R106 estimates e=3.08.

I looked at Sky and Telescope's Is Another Interstellar Visitor Headed Our Way? which shows the image and caption below.

Although it is impossible to be sure from only this photograph, the 0.65 m telescope looks like the distance from primary to the small secondary is only about 1 meter, maybe 1.2 at most.

Questions:

Why is this telescope so short?
How hard is it to make such a fast (and therefore deep) primary?

  Gennady Borisov with the 0.65-meter telescope he built and used to discover the new comet. G. Borisov


texts: Absolute magnitudes with dust extinction

texts:
I have a distant $z\sim3$ galaxy with absolute AB magnitudes $M_B$ and $M_I$, where the central wavelengths of the $B$ and $I$ passbands are $\sim0.4\,\mu\text{m}$ and $\sim0.8\,\mu\text{m}$, respectively.

If this galaxy was to experience $1\,\text{mag}$ of $V$-band extinction due to dust within it, where the central wavelength of the $V$ passband is $\sim0.5\,\mu\text{m}$, how do I work out what the absolute magnitudes $M_B$ and $M_I$ would be?

Thanks for any help with this issue.


texts: Can you see Pleiades and Sirius at the same time on the sky, from Greece?

texts:
I would like to know if you can see the Pleiades and Sirius in the sky at the same time, from Greece?
What time of the year would that be?

And I would be very greatful if someone knows an interactive starmap where I could see how that would look like.


texts: Semimajor axis variations in co-orbital moons

texts:
I've been playing with simulations of co-orbital bodies similar to Saturn's moons Janus & Epimetheus- horseshoe orbits where the two bodies are of comparable mass- and I'm seeing some very odd patterns that I can't explain.

What "should" happen is that the body with the slightly smaller orbit will eventually overtake the outer body; when their mutual interaction becomes significant compared to their interactions with the primary, the lower body is accelerated forward into a higher orbit, while the higher body is accelerated backward into a lower orbit, they switch positions around their average orbital radius, and then separate again.

What I'm seeing in simulation, however, is a little more complicated. When I simulate the Saturn+Janus+Epimetheus system, the moons' semimajor axis very slowly migrate farther away from their average up until they swap; right around the swap, the difference in semimajor axes is maximized, and they drift closer together for half a cycle and then farther apart again before the next swap. This creates "u" and "n" shapes on the plot of semimajor axis over time, with the plots for each moon in opposite phases.

For Janus and Epimetheus, the effect is very small, but increasing the masses of the moons amplifies it, to the point that the maximum separation in orbital radii that occurs during the moons' closest approach to each other is ten or more times larger than the "normal" separation during the middle of a cycle.

There is of course the possibility that this an artifact of errors in my simulation software, but I think that is unlikely given the following:

All other aspects of the Janus+Epimetheus system are reproduced perfectly- individual orbital periods are correct, average semimajor axes are correct, the frequency of the swaps is correct, and my simulation conserves energy to within less than 1 part in 1 trillion over a petasecond of simulation time (approx. 31,689 years).
The effect is perfectly regular, and simulated systems remain stable. If the effect was a result of simulation errors, I wouldn't expect them to behave so nicely.

My first thought was that perhaps the semimajor axis of each moon was being affected by the added gravity of the other moon when they are on opposite sides of the primary from each other, and when they approach more closely they won't be pulling each towards the primary as strongly. That, however, should result in a uniform expansion and contraction of both moons' orbits as they get respectively closer and farther away from each other. What I actually see is that the inner moon's orbit gets smaller while the outer moons orbit gets larger, and vice-versa.

So, is this likely to just be a simulation error after all (if it helps, I am using the 4th-order Hermite integration algorithm)? If not, what is the explanation for it? Has such an effect been documented before, and if so, where?


texts: Is a orbit possible where 2 objects going in opposite directions can intersect repeatedly with minor course corrections?


texts: Calculating the range of visible Right Ascension and Declination from specific location + time

texts:
Given a specific date and time, and coordination of a location on earth, how can I calculate (myself, or using a python package) range of visible RA and Dec?

For example (but I'm looking for a general solution and formula):

input:

date and time: 2019-05-01 23:00:00
earth coordinates: 32.0853° N, 34.7818° E

output:

RA min: ?
RA max: ?
Dec min: ?
Dec max: ?


texts: Is moisture harmful to a Newtonian Reflector?

texts:
It is often cold outside, especially in the evening. So when I take my Newtonian Reflector back in my house, it starts condense.

Can the water destroy my telescope? or the mirrors? Do I have to clean it?

My telescope is fully painted, but I'm afraid of the screws. Screws often rust.


texts: Do Pluto and Charon have unusual Lagrange points?

texts:
The usual examples of Lagrange points one most commonly encounters, Sun-Earth  and Earth-Moon Lagrange points, are examples of 3-body problems where $M_1\gg M_2\gg M_3$. The Pluto-Charon system, however, are much closer in their relative masses, so much so that their barycenter is outside Pluto's surface. From Wikipedia:

  Pluto and Charon are sometimes considered a binary system because the barycenter of their orbits does not lie within either body. The IAU has not formalized a definition for binary dwarf planets, and Charon is officially classified as a moon of Pluto.

How does this affect the orbital stability of the five Pluto-Charon Lagrange points?


texts: Does wavelength affect redshift caused by the metric expansion of space?

texts:
Does wavelength play a role in cosmological redshift? Do we see certain wavelengths affected less by expansion or notice any delay in arrival of certain wavelengths?

How accurate are any observations?


texts: Is there a web service providing orbital data of solar bodies (planets, comets, asteroids, dwarf planets)?

texts:
I would like to be able to connect to a web-service and retrieve JSON data containing information on current (as of today or so) orbits of planets, comets, asteroids, and dwarf planets in our solar system. (Even just planets would be fine but better if I can get info about asteroids, comets, and dwarf planets as well).
I would like to draw planets and their orbits accurately in a 2D animation of our solar system in an Android app in the X-Y coordinate plane as observed from the top of the Z axis.

Where can I find such public web service and how can I access it and get that data?

UPDATE
I tried using HORIZONS Web-Interface but out of all variables I need, I can see only "Semi-Annual Axis" information.  I need following information:

orbital ellipse angle in X-Y coordinate system (i.e 1.5rad or degrees or so)
motion direction (pro-grade or retrograde)
current position of planet/comet/asteroids on that ellipsis in degrees or radians, say 60deg from x (horizontal) axis
time it takes plant to go around the sun (I believe this is called orbital period)
eccentricity of ellipse (orbit)
semi-annual axis

These are the values that give me enough information to draw orbit (ellipsis) and correctly position planet on that ellipsis, and to calculate speed at which it moves around the sun in my 2D model.

So, I am not interested in inclination or any parameters that would be useful for a 3D model.

I can find this data online but it takes lots of searching since I have not been able to find a central place for it.  HORIZONS seems to be such place but out of variables I need (as described above), I can find there only semi-annual axis in HORIZONS.  I am surprised that I cannot fine a public web-service offering this info (REST or SOAP, or whatever, does not matter as long as I can fetch that data and process it for my needs from my application).

I apologize if my explanation is somewhat unclear, I am not an astronomer.

Hope that explains.


texts: What is the scale of things you can see with gravitational lensing?

texts:
I'm trying to understand the examples of gravity lensing (using the general relativity property of large masses to bend light like a lens).

Most of the examples I see are of some galaxy (presumably a large mass) between us and a star (or are at least vague enough to not specify.

Yet my intuitive understanding of the sky is that all the stars we see are relatively close, all entirely within the Milky Way galaxy (and many of the stars in our galaxy provide a general glow), and that other galaxies are far enough away that it is difficult to image individual stars. The only single objects large enough comparable to a galaxy would be a quasar. Is that right?

So what then is going on usually with examples of gravity lensing? I find it hard to believe the captions that say a galaxy is enabling seeing more distant stars. I would think one could only apply that concept to a star or galaxy to see something much further away and as big or much bigger. Could one really use a galaxy as a lens to see a star? I wouldn't expect a star to be behind a galaxy from us.

Also, whatever the objects themselves, what is the scale? If the lensing is of a galaxy done by a star, I'd expect the star to galaxy distance ratio to be well below 1:1000 (~width of Milky Way to distance to Andromeda). But for galaxy to galaxy or galaxy cluster to galaxy or quasar lensing what are the likely relative distances?


texts: Do parsecs scale as an inverse proportion with parallax angle?

texts:
high school student here.
In astronomy my teacher says that if the parsec angle ($\alpha$) of a star is 1/50 arcsecond, then that star is 50 parsecs away.
I'm not really convinced - calculation of distances from parsecs requires using the formula $$\text{distance in AU}=\frac{1 AU}{\tan(\alpha)}.$$
This means that distance is inversely proportional to $tan{\alpha}$. Distance is not inversely proportional to $\alpha$.

I know you can use a small angle approximation, but that feels to me as very dodgy - you can get fairly large discrepancies in the calculated distance depending on whether you use $tan(\alpha)$ or $\alpha$ to find the distance.

So, what is the best practice when calculating distances in parsecs? Is my teacher's approximation acceptable in "real life" astronomy?


texts: Filters for solar eclipse and solar feature viewing

texts:
I have

a Vixen ED100SF ED refracting telescope and
10x50 6.5 Bushnell Legend Porro Prism binoculars

and would like to outfit both with filters for solar eclipse viewing. I gather the basic setup simply involves finding an appropriate filter, that fits the equipment, but I'm new to this and the range of options is bewildering. Ideally I'd also like something that allows me to see solar features, but I gather that these can be quite expensive.

What are my options (or a good reference on options) for viewing solar eclipses and solar features? Would something like an "eyepiece" filter work on its own for safe viewing, or would my telescope require something different?


texts: What is Charon's ascending node in reference to?

texts:
NASA's orbital elements given for Charon's orbit are given at https://ssd.jpl.nasa.gov/?sat_elem. The ascending node is defined as being "measured from the node of the reference plane on the ICRF equator". For Charon, the plane being used is given as the "Mean equatorial (IAU equator)". I'm assuming this is Pluto's equatorial plane. What then is Charon's ascending node in reference to? Normally this would be ecliptic vernal equinox, however this obviously cannot be it.

Some sources have the ascending node as undefined, but that then begs the question, what is the argument of perihelion in reference to if the ascending node is undefined?


texts: Will biological process be disrupted under strong gravitational time dilation?

texts:
This question is raised by someone I've discussed with, after watching the movie "Interstellar", in which a clip showing Cooper and Amelia landing on Miller's planet near a super-massive blackhole and they experienced quite amount of time dilation due to the gravitational effect. They spent like 20 minutes there and when they backed to the station they found out that 23 years have passed on station (where Romilly waited them for 23 years to return).

It's okay for this conception to be implemented here I believe most people will understand. We will just think that yeah 23 years has passed but they just feel normal, just like a 20-minutes park walking. BUT, no one has ever experienced such a strong time dilation, how could you be sure Cooper "just feel normal"?

I remember reading a book which says if you are rushing directly to center of a blackhole, when you are very very close to the Schwartzchild radius, you may experience a whole lifespan of universe IN A SECOND, in other words, you are rushing to the real end of the world. I really doubt any life could 'feel' that second passing. Anyway, this is an extreme example.

So the question: will biological process behaves normal under certain gravitational condition? If it does, can we feel differently? Does our conscious get used to the normal time flow all life on earth are living with? more crudely, will we die due to the extreme time dilation caused by gravity (provided that gravity affect homogeneously upon us) and if so, how?

An interesting extra: If Romilly could observe Cooper from station and sees a near still 'slow-motion' picture of him, will Cooper conversely see a 'fast-motion Romilly' from the ground of Miller's planet?


texts: What is the most appropriate way to estimate the helium composition of a star?

texts:
Say we have to estimate the helium content in Proxima Centauri.

We begin by calculating the content of helium in the Sun (source): $24.85$% of $2.10^{30}$ kg.

Mostly all the energy is generated due to the product of nuclear fusion of hydrogen into helium by way of the proton–proton (PP) chain mechanism and the luminosity of Proxima Centauri is $0.00005\ L_☉$.

Thus, would it be a good approximation to say that the helium content is $0.00005 * 0.2485 * 2 * 10^{30}$?

Would you suggest a better way of estimating the helium content?


texts: Is there a complete edition of the Cambridge Catalogue of Radio Sources (esp. 3C) available digitally?

texts:
In radio astronomy, many common objects are refered to by their designation from the 3rd Cambridge Catalogue of Radio Sources, e.g. 3C 353.

For hobbyist purposes, I would like to create a catalogue for Cartes du Ciel that shows the location of the 3C objects. However, the catalogue exists in three versions, 3C from 1959 being the original, and 3CR (1962) and 3CRR (1983) updating the original.

Later surveys at different frequency bands used similar designations, e.g. 4C, 5C and so on.

Looking at the databases linked above, however, it seems that both only list the updates to the original catalogue (e.g. 3C 353 is listed in neither of them). Is there a digitized, comprehensive catalogue of all 3C objects? Or, even better, is there an archive where I could combine entries from several C surveys based on, say, 21cm flux?


texts:
I was recently reading an article on space.com about : What's the Temperature of Outer Space.

They said :

  Some parts of space are hot! Gas between stars, as well as the solar wind, both seem to be what we call "empty space," yet they can be more than a thousand degrees, even millions of degrees.

But here is my question - "How do we define temperature in outer space?"

We perceive temperature as the average translational kinetic energy associated with the disordered motion of atoms and moleculules, so how can the temperature be so high at some places, if there isn't any (or very, very few) molecules out there in vacuum?


texts: Why does the moon appear on cloudy night sky?

texts:
Now, I was looking the sky when I saw the Moon showing up in a sky completely cloudy, like this:

But a little different. The clouds were doing an eye shape. What does explain that phenomenon? Looks as if moonbeams drilled through the clouds!


texts: Can the Universe be spatially closed and expand forever?

texts: In the lambda-CDM model, the Universe is spatially flat, and will expand forever. Is possible that a spatially closed Universe expands forever?


texts: If two black hole event horizons overlap (touch) can they ever separate again?

texts:
Hypothetical question based on my understanding that two event horizons that overlap (touch) can't ever separate again:

Imagine a 1 billion solar mass black hole (so the event horizon is massive and very gravitationally weak) is travelling at a velocity of 0.9c through empty flat intergalactic space; now imagine an identical 1 billion solar mass black hole travelling at 0.9c but in exactly the opposite direction so the two are heading roughly towards each other. The black holes' paths, once all the space time warping is taken into account, aren't on a direct collision but the outermost edges of the event horizons will just 'clip' each other, ordinarily only overlap for a fraction of a nanosecond as these two bodies are travelling at such incredibly fast velocities and in opposite directions to each other.

So firstly, am I right in thinking that if two event horizons overlap they can never 'unlap'?

Secondly, what would happen to this incredible amount of momentum of each other the black holes? Would it just get instantly turned into gravitational energy? Bearing in mind when black holes normally merge, it happens very slowly as black holes slowly move closer and closer together over millions of years giving off gravitational energy as that happens, so not in a fraction of a nanosecond as in this case.

And thirdly, what would this look like? Would the event horizons remain fairly spherical and the radiated energy just insane or would they stretch and warp into a kind of long thin elastic event horizon as they shoot past each other and then over time slow down and snap back to each other?


texts: Which came first: black holes or galaxies?

texts: In other terms, did galaxies grow around black holes at their center?


texts: What's the relation between Einstein's Gravitational Theory and Dark Matter?

texts: I was reading an elementary book on dark matter (in fact, a historical perspective) and there were mentioned how the scientific community react to the idea of dark matter proposed as a solution to observed discrepancy between the actual mass of astronomical systems and the predicted mass from Newton's theory. I was wondering where Einstein theory stands in relation to dark matter, did it somehow predict it, or does dark matter prove the incompleteness of Einstein's theory? And what about dark energy?


texts: How can I obtain raw observation data of stars surrounding Sgr A*?

texts: Are there any publicly available telescope that offers the sensitivity to observe the stars surrounding Sgr A*? If not, is there any raw data available that I could download?


texts: star atlas with absolute magnitude and spectral type

texts:
Is there any star atlas someone recommends?

Then for some kind of star with a definite spectral type, we can know its absolute magnitude.


texts: How to find the distance between two quasars

texts:
I want to calculate the distance between two quasars of which I know the angular position and the red shift. Let $Q_1=(\alpha_1,\delta_1, z_1)$ and $Q_2=(\alpha 2,\delta 2, z_2)$ and suppose $z_2 > z_1$.

I know how to find the angular separation $\theta$ between them by means of the angular coordinates. But how to find the comoving distance (at epoch $z=0$) from them ? I know how to find the comoving distance from the Earth of the two quasars, can we find the distance between them using these two distances?

A related question is to find the redshift of $Q_2$ as seen by $Q_1$ at the epoch when $Q_1$ received the light emitted from $Q_2$ .

There is some standard method to solve this problem ?


texts: Are quasars simply AGNs that are viewed from a particular angle?

texts:
I found the picture below in an answer to the post "Was the Milky Way ever a quasar". The picture is published on a website belonging to "NASA/IPAC Extragalactic Database". In the answer that has 16 "thumbs up" it says:

  A quasar is simply an active galactic nucleus (AGN) that is viewed
  from a particular angle; see the picture below, in which quasars are
  labeled "QSO". This is really a remarkable figure because historically
  all of the names in the figure were thought to correspond to different
  types of objects, when really they all refer to the same thing!

There is another answer to the question "Difference between quasar and Active Galactic Nuclei?" where someone states that a quasars:

  main property is that it's galaxy axis points towards the earth and
  this way we receive its light and radio signals. That's why they are
  one of the most energetic AGN.

This answer has gotten six thumbs up. However, there is no reference to any source backing up that claim so it seems like private opinion.

Does the picture above represent the mainstream view among astronomers today or is it more like a private opinion of someone?
According to Wikipedia, ten percent of all observed galaxies are classified as Seyfert galaxies. Would we perceive all of those as "quasars" if we happend to observe them from the right angle? (Maybe there is also a luminosity requirement)
According to Wikipedia Markarian 231 is the closest quasar at a distance of 581 million light years. Also according to Wikipedia, the Circinus Galaxy is the closest Seyfert galaxy, only 13 million light years away. Would we perceive there to be many quasars closer than 581 million light years away, if we just happend to view the active galaxy from the right angle?
Could someone please show me the scientific investigations proving that the galactical planes of galaxies perceived to host quasars always lies within a certain range of observational angles as viewed from the Earth? These kind of investigations should be available if the claim that whether or not an active galaxy is perceived to be a quasar is almost totally depending on observation angle has a real backing and is not just speculation.


texts: Is the Milky Way Galaxy predicted to collide with a neighboring galaxy?

texts:
Topic

I've read several articles online that state that our galaxy will collide with a neighboring galaxy in the future, but there is no evidence about it happening.

Question(s)

  After reading a few articles on the topic, I started to wonder:

When will it happen?
Which galaxy will collide with our own?
Is it even going to happen?

(your post should answer all three)

It's a scary but oddly interesting theory, and a little enlightenment would be much appreciated! :)

Not a duplicate--

I'm asking if the possibility that any galaxy will collide with our own, not just one.


texts: What did the night sky look like 100,000 years ago?

texts: Question in the title. Would any constellations be identifiable?


texts: Do IRAS sources refer to star forming regions? I searched google and I came to know that it stands for IR astronomical satellite.But what do they search for? Please help me


texts: What determines which frequency range a radio telescope has?

texts:
My simple understanding is that a parabolic dish focuses radio waves to a receiver. It is obvious that any parabola focuses visible light and heat, so I assume that it focuses all radio wavelengths too. So evidently the design of the receiver determines which wavelengths it is sensitive to.

How is such a receiver designed in radio telescopes?
What properties of the receiver determine the wavelengths it is sensitive to?
Is the sensitivity range electronically adjustable?
Why aren't different receivers used so that the same parabola, for example the huge Arecibo dish, could be used for all wavelengths? (Or is it?)
What importance does the size and material of the parabola have for the sensitivity to different radio wavelengths?


texts: Perseid Meteor Shower Viewing events near Chicago

texts:
I'm going to be in Chicago during the weekend of Aug 11th, 2018. Can you suggest any viewing parties that are not too far away?

I'm very very amateur. I don't really know too much about astronomy, but do like being around people that do.

Please post any events that you know of like this one that was one about an hour away at Indiana Dunes State Park. https://www.trippy.com/a/5b65a86ae4b0e99fd1765e1b/507c527fe4b0c31db879944f

thank you


texts: How do stars change in size over their main sequence lifetime?

texts: I know that main sequence stars increase in temperature and luminosity as they age. However, I do not know about how they change in size. Do they expand? If so, why do they expand? Do they grow very much? I’ve tried to search up the answer, but details were limited, so I would appreciate an answer here.


texts: How do I calculate the velocity dispersion of a galaxy given an integral field spectrograph?

texts: I couldn't find on the web how to get the velocity dispersion and velocity maps of galaxies from the 3D data cube that I get from the integral field spectrograph.


texts: Why are the ALMA receivers' ADCs only 3-bits?

texts:
above: Table 1 from Performance Highlights of the ALMA Correlators

The ALMA receivers use 3-bit ADCs for what would seem to be to be a high dynamic range application needing much finer quantization to get anything useful.

Then I found these sentences within the abstract of ADC bit number and input power needed, in new radio-astronomical applications:

  Abstract- For the most part, so far radio astronomy observations have been performed in protected frequency bands, reserved by ITU for scientific purposes. This means that, ideally, only the amplified equivalent system noise is present at the end of the receiver chain (i.e. the ADC input). So, typically, only a few bits are necessary to describe the signal (VLBI signals are digitised with only 2 bits), but today astronomers, in order to get more sensitivity and to boldly observe where no one has observed before, would like to study the radio sky even outside the protected bands...

And I even found a 1-bit ADC in Performance Measurements of 8-Gsps 1-bit ADCs Developed for Wideband Radio Astronomical Observations.

I think I am just missing something obvious, but I can't understand how a measurement requiring high dynamic range gets by using few-bit ADCs.

edit: Is it possible that the actual conversion of analog to digital is done to a far higher precision than suggested by the number of bits?


texts: Definition of Rotation versus Revolution

texts:
Let there be a planet for which the duration of a solar day is equal to a year around the sun such that:

1 "day" = 1 "year" = 100 Earth days (2400 hours or 8640000 seconds)

Then, would we say that the rotation of the planet is 0 or equal to 100 days?


texts: If Pluto is tidally locked with Charon, is its rotation still slowing down?

texts: I know that a a planet or moon's rotation will slow down or speed up until it is tidally locked with a body, is Pluto's rotation being affected by the sun or is it being forced by Charon just to to be tidally locked with Charon?


texts: Who or what will the Arecibo message reach?

texts: The Arecibo Message was aimed at the globular star cluster M13, but as far as I read, M13 will move out of the original position and thus not be able to receive it. So, who or what will be able to receive the message?


texts:
Today I could roughly see Saturn with ring structure for the first time with my Seben 114-1000 telescope.

Then later I was dealing with Mars and found out that I must do something wrong, as with no eyepiece I could really see this planet closer.

So, whether I take 20, 12.5, or 6 mm eyepieces, the size as I set sharp focus remains a small light orange circle of solid uniform colour.  The app sky view shows for today 64% clear view, maybe an explanation why I can't see more details.

But is this possible to see Mars filling in the view field? And why there is no recognizable difference between 20 and 6 mm - do I need them in combination with something else?
Here it says, even the polar caps of Mars should be recognizable with this telescope model scale.

UPD

So far, it looks like that with up to 6 mm I can zoom in fairly well to such details as:

large moon craters
Jupiter (seen as small white illuminated blob) with the Galilean moons (seen as  stars)
Solid white-coloured Saturn with its ring and some space between planet and ring visible

Mars keeps being small to medium yellow-orange disk of solid color.

Using 4 mm gives little bit more zoom but less detail.

However, I can see with 6 mm that Moon contours are slightly jittering, maybe through light refraction in the night air (all observation done from the ground on a field). The temperature in the night has been 21°C.


texts:
It has been argued that it is not possible to trace back the remains of the progenitor star of the Sun (sometimes called Coatlicue), which, being estimated in the mass range of 30 $M_{\odot}$, should have left a black hole after its explosion as a supernova, nor is it possible to find the sister stars formed from the compression of the same nebula:

https://astronomy.stackexchange.com/a/10729

https://phys.org/news/2012-08-solar-genealogy-revealed-meteorites.html

because "[The Sun] has completed many Galactic orbits [since its formation]. Not only that, but it could have migrated in orbital radius as well." and "These twin stars did not influence its development nor that of its planets [and] are now dispersed in the Galaxy and cannot be identified".

Why did the supernova's remains and the sister stars not remain gravitationally bound together? Why did they not share the same angular momentum of revolution around the Galaxy?

Why did these massive bodies formed from the same nebula not become a bound system among them and with the supernova's remains, like the Sun and its planets in the Solar system?


texts: When will we be able to detect Earth-sized exoplanets?

texts: I have heard that the only planets able to be seen outside the Solar system are Jovian-sized planets with the occasional detection of planets three times the Earth's size. But, as far as I know, we haven't seen any Earth-sized (due to the range of our telescopes?) extrasolar planets. When will we be able to detect them?


texts: Using a refractor telescope 5" aperture, 900mm focal length for terrestrial viewing

texts: Can I use a refractor telescope 5" aperture, 900mm focal length on an equatorial mount for terrestrial viewing.


texts: How to deal with data that's billions of years old?

texts: We look at galaxies that are billions of light years distant and draw conclusions based on the red shift of the light. How is the age of the light accounted for. We see the reality of a galaxy perhaps a billion years ago. Why not conclude that redshift is related to time instead of distance?


texts: I saw many moving stars moving at high speed and disappearing at the middle. What could it have been?

texts:
Time : 6:30 to 1:00 P.M.
Location :  8°52'42"N 38°48'49"E
Date : October 28, 2020
I saw stars moving one after another and there were many of them. After moving some distance they vanished one by one. it was not a shooting star as they move at high speeds. I am sure it is not satellite because they were many and they were quite fast.


texts: Did the Babylonians believe in the Heliocentric version?

texts:
It's mentioned that the Babylonians decreed that there were exactly 360 days in a year. This statement was based upon the utilization of 360 as a standard (based on the sexagesimal system) i.e., they divided the circle into 360 equal parts.
Although, there is a mention about the Sun making one full circle across the sky in one year (as estimated by them), but, how were they able to measure it? And did they believe in the Heliocentric version of the solar system?


texts: Why Is The Expansion of The Universe Allowed To Move Faster Than Light?

texts:
Why is the expansion of the universe allowed to faster than light if everything that moves within a vacuum can't reach the speed limit of light?

I've read that astronomers looked at the distance between galaxies but I'm still not understanding what's going on. If objects in the universe can't go faster than the speed of light in a vacuum, how are galaxies (which objects within the universe) moving away from each other faster than light?


texts: How does science knows what is the early universe?

texts:
I've read about Hubble seeing the brightest quasar in the early universe. Question is, how does science knows what is early and what is late?

Please correct me if I'm wondering, but after the big bang, everything went in all directions equally. How do we know what direction is the center of the big bang? And continuing on the line of thought, is "early universe" considered closer to the origin of the big bang or further away?


texts: Does a replacement exist for Dexter? (A tool that reconstructs data from a pdf of a graph)

texts:
Dexter was a tool from the German Astrophysical Virtual Observatory, made by the Center for Astronomy of Heidelberg University (ZAH). It was able to reconstruct data from a graph if you provided a pdf or an image of that graph. It relied on Java, and since browsers have stopped support for Java it no longer works.
Website for reference: https://dc.zah.uni-heidelberg.de/dexter/ui/ui/custom
Does anyone know of another tool that does the same thing?
Thanks


texts: Can matter fly directly into a black hole and avoid the accretion disc?

texts:
From what my non-scientifically-trained mind understands, matter typically enters a black hole via getting caught up in the accretion disc going faster and closer to the black hole until it passes the event horizon, the point of no return. Using Earth as an analogy for a black hole, I assume the accretion disc would orbit around the equator, similar to Saturn's rings, and the matter might eventually crash into Brazil or somewhere along the equator?

My main question is, can matter simply do a straight shot into the black hole at a higher latitude, say straight into New York City, avoiding the disc altogether?

As you can probably tell by my simplistic analogy, I am a complete layperson. While fancy jargon and artistic looking equations might be expected (which is fine), please also dumb it down to an average intelligent business administration major's level of scientific understanding.


texts: Are there any physical phenomena that destroy information?

texts: Are there physical phenomena, such as wave function collapse or black holes, which irrevocably destroy information about their prior states?


texts: Determining distance from semi-major axis and eccentricity

texts: I am trying to obtain the distance covered by an object in orbit around the earth within a specified amount of time past its perigee passage. The object is in an elliptical orbit with eccentricity 0.2 and has a semi-major axis of 9600km. Where would I look to find the objects position 90 mins after it passes its perigee? Thanks for any advice


texts:
I tried to find the habitable planets, thus I suppose to calculate the effective temperatures that incident on the planets from their host stars, I used

$$
T_\mathrm{eff}=\frac{L}{4\pi\sigma_\mathrm{SB}R^2}
$$

but I don't know how to calculate the effective temperatures in polar regions?


texts: What should I consider in order to accurately calculate the moon phase?

texts:
Although this question apparently addresses the topic, it does not tell appropriate factors involving the moon phase.

I found two distinct algorithms. this one and also this one. I am less than a newbie, less than a layman, to astronomy - but I am not to programming, but I would like to skip testing the algorithm for a couple of years before choosing either.

After unsuccessfully finding a comparison of different algorithms, I'd like to know of either of these algorithms is less error-prone, or if there is someone less error prone-to these (I know there is no stuff like a perfect algorithm, but I'd like to have a good one), or having appropriate ways to judge an algorithm.

So my question comes like this:

Do these methods have a small error, enough to accurately calculate the moon phase?

Or...

Can you give me the most precise algorithm -or concepts / underlying theory I should know- astronomers would use as of today to calculate the moon phase?


texts: If getting shined on by a gamma ray is an extinction level event how do we detect them?

texts:
Cosmic gamma ray bursts are seriously powerful and do a lot of damage. So how did we measure gamma ray frequencies without getting fried?

I have two possible explanations:

We never detected them directly but calculated their frequency from secondary radiation from the surrounding gas that was ionized.
We get a direct hit but the source is so far that beam divergence will be large enough for only a few photons per square meter to hit us, so there's not enough energy to be destructive.


texts: What are some of the best astronomy gatherings in the USA?

texts:
I'm part of a local astronomy club but the results from that have been vastly underwhelming with a membership count of around 50-200 active people at any given time. All that being said- I guess my question is for 2019 what are the biggest and most informative gatherings in terms of astronomy? Anything you know of from lectures to imaging to stargazing would be wonderful.

More specifically:
I was considering spending the 250 dollars to go to the Northeast Astronomy Forum this April 4th and 5th and the Northeast Astro Imaging Conference. It seems to be one of the larger scale events that happens anually. Is this a good event for beginners? Heres a link to their site:

http://www.rocklandastronomy.com/neaf.html

http://www.rocklandastronomy.com/neaic.html

If youve been Id love to hear from you. If this is off-topic I apologize, but the only other place I had to ask was our clubs forum and it has virtually no traffic.


texts: Information speed and our view of the cosmos

texts:
It is common knowledge that when we look at the stars we look back in time. And most stars are thousands of lightyears away from each other. The information that photons cary about the position of a stellar object can be really deceiving about their true position.

Do astronomers take lightspeed into account when they map stars ? If so, how do they figure out the true position of a star.

With all that in mind do galaxies really look like what we see in our photographs? If a galaxy has a diameter of bilions of lightyears, what we see could be a really skewed and warped image of the real galaxy. Can we computationaly reverse the warping ?

EDIT
Image 1 and 2 explain what I mean by "Warping". Stars closer to the core move faster and in combination with their distance from the viewer, information by light should give the impresion of a more "spiraly" galaxy as it whould take less time to reach the observer. Image 2 whould be the "true" shape of the galaxy if the effect could be reversed (I applied a counter clock-wise warp distortion)

An other factor of false position are gravitational lences. Although gravity from black holes do give the maximum distorsion, gravity from other stars could divert light reaching us. The diversion is negligent but can get big for astronomical distances (Image 3). Am I right ? If I am, do Astronomers even bother to make simulations to map the known sky as it whould be in real time ?


texts: How does the Hubble Space Telescope "Drift and Shift"?

texts:
The Phys.org article Mystery of the universe's expansion rate widens with new Hubble data says:

Astronomers have been using Cepheid variables as cosmic yardsticks to gauge nearby intergalactic distances for more than a century. But trying to harvest a bunch of these stars was so time-consuming as to be nearly unachievable. So, the team employed a clever new method, called DASH (Drift And Shift), using Hubble as a "point-and-shoot" camera to snap quick images of the extremely bright pulsating stars, which eliminates the time-consuming need for precise pointing.
"When Hubble uses precise pointing by locking onto guide stars, it can only observe one Cepheid per each 90-minute Hubble orbit around Earth. So, it would be very costly for the telescope to observe each Cepheid," explained team member Stefano Casertano, also of STScI and Johns Hopkins. "Instead, we searched for groups of Cepheids close enough to each other that we could move between them without recalibrating the telescope pointing. These Cepheids are so bright, we only need to observe them for two seconds. This technique is allowing us to observe a dozen Cepheids for the duration of one orbit. So, we stay on gyroscope control and keep 'DASHing' around very fast."

I understand that the technique skips the use of guide stars, but I'm wondering exactly what does happen. What does the word recalibrating mean in "..move between them without recalibrating the telescope pointing..."
Do they just execute a blind change in direction using Hubble's attitude control system of gyros and reaction wheels, expose, and move on to the next field, and hope the target object was imaged?
For a little background see Hubble's one-gyro mode; how does it work for attitude control, stabilization, and slewing? and answer(s).


texts: Would exoplanets like those of Kepler-70 possess magnetic fields?

texts:
The B-type subdwarf, Kepler-70  (aka KIC 5807616) is believed to have two planets orbiting it, at average distances 0.006 AU and 0.0076 AU (source). Due to their proximity to their parent star, they are almost certainly tidally locked to it (although I don't know if tidal forces between the planets themselves might counter this, since they apparently pass within a few hundred thousand miles of each other and have short orbital periods.)

Now, the "day side" of each of these planets would have surface temperatures exceeding that of the Sun! (i.e. > 6000 K.) If, indeed, they are tidally locked, we might also expect a significant temperature difference between the "night" and "day" sides. The planets are believed to be the rocky/iron cores of gas giants that were engulfed when their parent star expanded. If I have understood the paper by Nordhaus et al. correctly, the gas giants would have had to be significantly larger than Jupiter. Certainly, however, the Chthonian planets that they turned into are now hot enough for metal/rock to be evaporating on at least one side, and possibly both. It would seem unlikely that they would be mostly solid.

If this is correct, then:

We have two planets, each mostly liquid/gaseous, with significant temperature differences between different regions. So it looks to me as though there would be convection.
If the planets have significant metallic content, they would also be conductive.
They are certainly also rotating, at the speed required to keep one side continuously facing the star.

Am I correct, therefore, that they probably have magnetic fields generated by the dynamo effect? Or is there some factor, possibly related to the extraordinarily high temperatures involved (or my own lack of knowledge :-) ) that would imply otherwise?

Sources:

J. Nordhaus, D. S. Spiegel, L. Ibgui, J. Goodman, A. Burrows, Tides and tidal engulfment in post-main-sequence binaries: period gaps for planets and brown dwarfs around white dwarfs, Monthly Notices of the Royal Astronomical Society, Volume 408, Issue 1, October 2010, Pages 631–641, https://doi.org/10.1111/j.1365-2966.2010.17155.x
Charpinet S. et al. A compact system of small planets around a former red giant star. Nature vol. 480 (2011)
Bear, E., & Soker, N. (2012). A tidally destructed massive planet as the progenitor of the two light planets around the SDB star KIC 05807616. The Astrophysical Journal Letters, 749(1), L14.


texts: Can any type of stars be contact binaries?

texts:
In this article Shortest distance between stars it is mentioned that

  Some double stars can actually be in contact with each other.

  This occurs when a red giant star has a white dwarf star companion in
  a very close orbit.

However, this isnt the case

Hottest, heaviest touching double stars

  In the case of VFTS 352, however, both stars in the system are of
  almost identical size. Material is, therefore, not sucked from one to
  another, but instead may be shared.

My question then is, can any type of stars be contact binaries, or are there some limitations in the type of stars for contact binaries to appear?


texts:
A spaceship with mass $m = 25\:kg$ is moving toward a red giant with $M = M_{sun}$ and $L = 1000\:L_{sun}$ (L: Luminosity). It is moving on a line (so this is not an orbital question). When it gets too close to the red giant, it's sensor open its solar sails (they get and then reflect the light completely), hence the radiation pressure help it to escape from the red giant (on the same line). Find the area of solar sail. (It is from National Olympiad of an Asian Country)

My problem is with the radiation pressure formula. for solving this problem, somewhere said the radiation pressure is: ($c$: light speed, $\sigma$: Stefan-Boltzman Constant)

$$P= \frac{4\sigma T^4}{3c} = \frac{4}{3}\frac{L}{4\pi D^2}$$
Where $T$ is the equivalent Temprature in distance $D$.

Somewhere else said:

$$P= \frac{4\sigma T^4}{c} = 4 \frac{L}{4\pi D^2}$$

In another book it is:

$$P=2 \frac{4\sigma T^4}{c} =  2\times 4 \frac{L}{4\pi D^2}$$

2 is because it gets the light and then reflecting it.

At the final:

$$P\times A = \frac{GmM}{D^2}$$

I want to know what is the right solution for calculating radiation pressure?

I must say that the below picture is also with the question but in the books, it was used for another part of the question which is not my problem and I didn't mentioned it above.


texts: How would you define "space" in GR (General Relativity) and in QFT (Quantum Field Theory)?

texts:
Is there a univocal definition of space that can be accepted in GR and in QFT?
In GR space is actually "Spacetime", a 4-dimensional entity that includes time. In this case it's impossible to separate the container from its content, in fact Spacetime between Superclusters of galaxies, is not completely empty but contains a very low density of particles, mainly hydrogen and helium in the form of plasma, electromagnetic radiation, neutrinos, dust and cosmic rays. Spacetime also warps due to different forms of energy so must be considered in a 5-dimensions manifold.
In QFT empty space is a state more than a container and is never truly "empty" (it's filled by transient electromagnetic waves and particles popping in and out continuously), so vacuum state can be defined in various ways:
QED (Quantum Electro Dynamics) vacuum is the ground state of the electromagnetic field when the fields are quantized.
In QCD (Quantum Chromo Dynamics) is characterized by non-evanescent condensates such as gluonic condensate and quarks condensate.
False vacuum, a hypothetical vacuum that is somehow not entirely stable and could decay into a lower energy state.
Quantum foam, defined as spacetime fluctuations of unimaginably small scales of the order of Planck Length.
How can we unify this multiple description of Space?


texts: We know that it is a commonly heard claim that in every black hole there is a universe. It is also believed that the universe is expanding. If we use both of these, can we conclude that every black hole is expanding?


texts: Does "spectral type" and "stellar classification" refer to the same thing?

texts:
Should the tags stellar-classification and spectral-type be merged? has been asked in meta two weeks ago. It requires some careful consideration but so far no response has been forthcoming, so I'm asking this question here to get a definitive answer that can then be applied there.
Wikipedia's Stellar classification begins

In astronomy, stellar classification is the classification of stars based on their spectral characteristics.

and while there is one subsection labeled simply Spectral types to my untrained eye it looks like there may be other classification schemes besides O, B, A, F, G, K, M, L, T.
Question: Do the astronomical terms "spectral type" and "stellar classification" refer to the same thing? Or at least always when applied to stars? Might "spectral types" also refer to a way to classify asteroids?


texts: How bright could Gliese 710 appear in 1.35 million years?

texts:
I read this article on Gliese 710 that stated it would come within about 1.1 ly (+/- 0.577 ly) of earth in 1.35 million years. Wikipedia states it would have an apparent magnitude of 0.5, and the original paper abstract states it would have a -2.7 apparent magnitude.

From reading the list of brightest stars, apparent magnitude of 0.5 would make it about the 10th brightest star, and -2.7 would make it the brightest (besides the sun).

I'm curious about the discrepancy between these sources.

How bright would Gliese 710 appear if it were at the near end (0.523 ly) and far end (1.677 ly) of the expected closest approach to us?


texts: Integral Field Spectroscopy vs Fabry-Perot Interferometry

texts:
Why are Fabry-Perot Interferometers (FPIs) and Fourier Transform Spectrographs (FTSs less common than Integral Field Spectrographs (IFS) nowadays? My understanding is that:

With FPIs and FTSs you have to scan your spectral range over time. This (A) Takes more telescope time, and (B) Is affected by seeing variations over time (i.e., different wavelengths end up having different PSFs).
Limited spectral range. As a result, you have limited information about the observed object.
I was also under the impression that reducing FPI or FTS data is more complex. However I do not have a reference to support this claim. It this true? If it is, can someone explain why?
For some reason FPIs use Image Photon Counting System (IPCS) instead of CCDs. Why is that? Does this somehow make them "worse" than IFSs?

What other advantages does IFS have over FPI and FTS?


texts: Does blackhole merging break their event horizon segregation?

texts:
So far as I know, if we have 2 blackholes A and B, both having their own event horizons, then what's inside the event horizon of A remains completely unknown to us, and same for what's inside the event horizon of B.

Now, suppose A and B coalesce (merge). So we now have a single blackhole C, with its own new event horizon.

This would mean that what was inside the event horizon of A now "knows" what is inside the event horizon of B.  Would this mean that all event horizons of all blackholes are actually "linkable" together since they might all merge at some point?

Edit: After watching the suggested video, I can try rewording the question:

Suppose Alice and Bob are outside blackholes A and B.

Alice jumps into blackhole A, and Bob jumps into blackhole B.

Bob never sees Alice crossing the event horizon (according to the video below; it's the same Bob than in the video: he cannot see Alice crossing the event horizon until "t=infinity"), but Bob crosses the event horizon of blackhole B (not part of the video).

Let's say that the two blackholes are also falling to one another so let's say they are "now" (that concept is probably misplaced) merging. So the two event horizons now become one.

Are Alice and Bob reunited despite Bob never seeing Alice cross the event horizon of A? How would such situation (2 blackholes) be drawn using the diagram in the video in answer below?

This might be related to Do black hole singularities actually merge? and What happens to the information on the event horizons of two merging black holes? but I'm unable to get my answer from there... but I'm only a computer engineer, I'm not a physicist, so the answer might be lying there and I don't see it.


texts: Calculation of Eccentricity of orbit from velocity and radius

texts:
For a school project I am making a gravitational computer model. As I want to Milankovitch cycles, I need to calculate the eccentricity of an orbit after the model has completed its simulation.

I have thought about calculating the eccentricity using the aphelion and parohelion height, but these are not available as it is a simulation and as it therefore stores the data for one point on the ellipse.

The data I have after the model has completed:
X,Y,Z coordinates of points that it has saved
X,Y,Z velocities of points that it has saved

I have been able to calculate the semi-major axis using the vis-viva equation. I have tried to get the value using the vis-viva equation but I got stuck as I think I need at least the Aphelion or Parohelion to calculate the eccentricity.

My question: Does anyone know how  to calculate the eccentricity at any given point in the orbit using the velocity and coordinates (without simulating all points around it)?

If not, does anyone maybe have a better idea/suggestion that I can try to gain the eccentricity values?


texts:
Are there commonly used names for objective directions within a spiral galaxy?

Central and peripheral describe objective directions, because the observer's frame of reference doesn't affect whether something is closer or farther from the middle.  "Clockwise" or "counterclockwise" don't work, because they depend on which side you're viewing from.

Are there names that can describe one object's location relative to another object, either in the galaxy's direction of spin or against it?  Or words to describe whether something is farther or closer out along a particular arm of a spiral galaxy?


texts: Would alcohol wiping damage multi-coated eyepiece coating? How else can I take COVID precautions on the eyepiece?

texts:
My small group of friends will be wearing masks. But could COVID-19 be spread by eyes/eyelashes touching the eyepiece? If so, what precautions should I take?
I have a small Starblast scope with an Orion E-Series 7-21mm Zoom Eyepiece which "consists of six lens elements, which are fully multi-coated with anti-reflection coatings for bright and clear views, rich in contrast." Would 70% alcohol wipes damage the coating? Would it matter if it's isopropyl alcohol or ethanol alcohol?


texts: Date and Time of Year given Sun's Right Ascension

texts:
Can anyone tell me how to calculate date of year given a specific sun's Right ascension (RA)?
I know RA at Vernox (21 March 00hh:00mm:00ss)is 0 degrees.
How can I get date and time for a given RA, say 1h 29m 6.69s?
Thxs vm


texts:
I'm trying to get some data about the moon from SIMBAD (specifically, radiation spectrum in certain wavelengths), and I encountered an unexpected problem: I can't find the Moon.
In SIMBAD, I can search using several different methods. Since I'm looking for a specific object in the sky, I thought the best method would be just to search for the moon by its name, using "Query by identifiers".
It may sound silly, but the problem is I don't know what is the nomenclature of the moon, nor how to find it. Using the obvious guesses ("Moon", "Luna", etc.) gave me nothing. What am I missing here? Does identifier mean something else than what I thought?
P.S. This problem also occurred to me while searching for the Sun, which signals to me that I'm probably missing something critical here...


texts: Do astronomers and astrophysicists more often use diameters or radii when discussing about planets, dwarf planets, exoplanets and stars?

texts:
Mathematicians much more often use radii over diameters when discussing about circles and spheres, because in mathematics the radius is more fundamental than the diameter (the sphere is defined using its radius).

But what about in astronomy? Do astronomers and astrophysicist more often use diameters or radii when characterizing planets, dwarf planets, exoplanets and stars?


texts: What could be a star-like object that shines at daytime for few seconds? What's the probability to see it?

texts:
Some years ago I looked up to the blue sky, around 2 PM, and I saw, for 2 or 3 seconds, something like a star fading out.
Could it be a Supernova or another phenomena? Is it highly possible?


texts: What causes the dimensions of a star increase when its hydrogen fuel is exhausted? For example, the Sun is expected to increase its radius 250 times. What causes this if its temperature is expected to fall? How gas can expand if the temperature falls?


texts: If the Milky Way galaxy is as big as planet earth, how big is Earth?


texts: Are asteroid complexes and types the same?

texts:
Very novice astronomer here. I keep seeing terms like "S-Complex" thrown around when talking about asteroids, but no matter how much Googling I do I can't figure out what that exactly means (every article just writes about it assuming the reader knows the definition).

Finally I looked up a free textbook and read the section on asteroids. In Composition and Classification it talks about there being three "types" of asteroids (another term I have seen thrown around): C-type, S-type, and M-type. (I've also seen them referred to as Type C, Type S, etc.)

So are the words "type" and "complex" just synonymous words to classify asteroids? If not, what does "_ -Complex" mean when talking about things like S-Complex asteroids?

Thanks!


texts: Is it possible to overcome the problem of blind spot(s) of current gravitational wave detectors?

texts: If gravitational wave pass through specific points (which are know as blind spots) current GW detectors aren't able to detect the passed waves so over the time will we be able to completely overcome this problem or will it be there always upto certain extent?


texts:
I'm trying to compute solar eclipses for a long period of time, see Back-predicting solar eclipses

Initially, I wanted at several points on Earth whether an eclipse was visible at some point in time.
I have since changed the approach and I will now compute whether an eclipse could be seen from the center of the Earth at a given moment. Later, after knowing roughly the time of the eclipse, I will iterate over a grid on Earth to check where it could be actually seen.
So far, my first question is: is it right to check for eclipses at the center of the Earth?

Secondly, I wrote a small code in Python to test how good I am at predicting eclipses, checking against https://eclipse.gsfc.nasa.gov/SEsearch/SEsearchmap.php?Ecl=01501207
for these coordinates:

Lat.: 27.4782° N
Long.: 109.9299° W

#!/usr/bin/python

from skyfield.api import load, Topos
from skyfield.positionlib import ICRF

planets = load('Ephemerides/de422.bsp')
ts = load.timescale()

earth = planets['earth']
sun = planets['sun']
moon = planets['moon']

time = ts.utc(150,12,06,23,49,21.9)
place = earth + Topos('27.4782 N','109.9299 W')

observe_moon = place.at(time).observe(moon).apparent().position.au
observe_sun = place.at(time).observe(sun).apparent().position.au
distance = ICRF(observe_moon).separation_from(ICRF(observe_sun))
distance = distance.degrees

print distance

The distance I obtain is above 10 degrees, while at the time provided, if I did things correctly, the eclipse was at its maximum at those coordinates. What is wrong in my code or my understanding?


texts:
Why black holes look black to us?
The question might sound weird but my head stuck in it. As we know objects that fall into the black holes seem to remain unchanged in its event horizon due to the nature that lights cannot escape from it. For example, in the below image (image with white astronaut) of the black hole we will see the astronaut remains in its event horizon.

Now, we also know that the black holes are constantly pulling all the interstellar objects & meteors around it, thus if these objects are grabbed by the black hole, we are supposed to see many of these objects remain unchanged on its event horizon surface hence the color of the black hole is not supposed to be black instead the distorted color by the objects (i.e. below image). But still we see its color a pure black in most of the representations and photos.

So why do we see black holes black instead of distorted colors of the space objects?


texts: How to deal with shifting CCD bias (zero exposure) levels?

texts:
The median values of the bias frames coming from our teaching telescope shift up or down with each capture. For example:

import glob, numpy as np, astropy.io.fits as fio, matplotlib.pyplot as plt

data = np.zeros((10,255,765), dtype=np.int)
for i, f in enumerate(glob.glob('bias_*.fit')):
    data[i,:,:] = fio.getdata(f)

print(np.median(data, axis=(1,2)))

[ 106.  106.  108.  108.  110.  108.  105.  106.  107.  107.]

Now, during data reduction we average combine our bias frames. We also reject outliers (either minmax or sigmaclip). But if some of our frames are "shifted" higher than others, wouldn't this skew the averaging (and especially the outlier rejection) so that we might be introducing more noise during averaging?

To give you an idea of the nature of this fluctuation, I did a quick analysis: read 10 bias frames, then iterated through them, subtracting each frame from the median combination of all other frames. This gives me something like "noise frames", showing only how much each frame deviates from the median of the rest. Below is the code and the distributions of two of these noise frames.

noiseframes = np.zeros((10,255,765), dtype=np.int)
otherframes = np.ones(10, dtype=np.bool) # just a mask
for i in xrange(10):
    otherframes[:] = True
    otherframes[i] = False # exclude current frame from mask
    median_of_others = np.median(data[otherframes,:,:], axis=0)
    noiseframes[i,:,:] = data[i,:,:] - median_of_others

def plotdist(i):
    frame = noiseframes[i].flatten()
    x = np.arange(np.min(frame),np.max(frame)+1)
    y = np.bincount(frame-np.min(frame))
    plt.step(x, y, label=str(np.median(data[i,:,:])))

plotdist(4)
plotdist(3)
plt.xlabel('Difference from median [ADU]')
plt.legend()
plt.show()

All of the noise frames have near-perfect gaussian distributions with pretty much constant FWHM, it's just the central value that shifts around with each frame.

This leads me to a couple of conclusions: firstly, this shift truely is a shift in median value and not just a result of something like cosmic rays; secondly, the fluctuation is an additive effect and so not due to a change in gain due to a difference in voltage or something like that.

I also could not find any correlation with temperature or time since last readout, both of which I suspected for a moment.

Perhaps it's slight changes in readout time?

I couldn't find any mention of this kind of artefact in my own course notes. It's also not mentioned anywhere online that I can find.

Can any of you help me to a) identify the possible sources of such a fluctuation and/or b) provide a proven strategy for dealing with this during data reduction?


texts: Did the Juno mission locate Jupiter at the same precision as Saturn (4 km)?

texts:
Juno's low perijove should be great for measuring Jupiter's gravitational effect on Juno's orbit. Cassini together with VLBI has measured Saturn's location to within 4 km.

Will Was Juno be able to be measured by VLBI to determine Jupiter's location and orbit at a similar precision? At a precision useful for mapping the Solar System outside of the Kuiper Belt?


texts: What was Bradley's observation concerning abevation of light?

texts:
As far as I understand, the effect of parallax exhibits itself in the shift of a star relative to the background stars on photographs.

As for the aberration of light, how is it reflected? It would not cause the relative shift of a star in a background, right? Because all the stars (the foreground one and the background ones) will be rotated as a whole, right?

So, what was Bradley's observation?


texts: What is the composition of the Solar Wind?

texts: Is the composition of the Solar Wind known? I'm especially interested in the heavy metals present. Most accounts deny the existence of anything other than electrons, protons, and some alpha particles; but it seems to me that if we can identify most of the known elements in the Sun's photosphere, then they are likely to exist in the Solar Wind at some concentration. If not, what is the detection limit (ppm? ppb? ppt?) we have so far established? Have any limits been established for atom clusters (dust?)? As a secondary question, I guess that (neutral) hydrogen and (negative hydride) H(-) do not exist in the solar wind due to its temperature being too high. Is this correct, and if so, is it true throughout the heliosphere or does the solar wind cool enough for the formation of bound species?


texts:
Motivated by this question about a stranded submarine in the Jurassic I would like to know:

Is there anything remotely left of our current skyline if we go back or forward millions of years so that the stars rotating around the milky way covered a measurable distance ? I would expect that the proper motion of a star causes an irreversible drift out of the original position. Are there any stabilizing factors which may allow that some star configurations are still discernible ?


texts: How good is the HST compared to ground telescopes?

texts: The Hubble Space Telescope (HST) is a remarkable device. It produced (produces) images whose resolution and clarity were a step change ahead of anything that could be captured from the ground. Is this still the case, or have adaptive optics and ever larger mirrors caught up with the image quality the HST can achieve? Is it still true that the HST's images are better than anything produced on Earth?


texts:
We have discovered about 2500 radio pulsars. Part of them show pulsations in other bands like X-ray and gamma ray. The radiation mechanism remains unclear now.

I wonder whether there are radio pulsations caused by the spins of white dwarfs. It is the best to give reference papers.


texts: Could a moon orbit an Earth size planet in such a way as to create a solar eclipse lasting for several weeks from start to finish?

texts:
Is it theoretically possible for a  moon to orbit a planet (let us say Earth size, mass and solar distance) in such a way as to create a planet-wide solar eclipse lasting several weeks?

These are my assumptions (some of them likely flawed) about the circumstances under which this might be possible:

To simplify the problem, let us assume that there is no planetary axial tilt and that the moon has an equatorial orbit.
The orbital speed of the moon would need to be quite slow in order to stay in position for long enough to create a graduating eclipse over several weeks.
In order to create a planet wide eclipse, the moon would need to be quite large and/or relatively close to the planet. Is this circumstance feasible if the mass of the object were low compared to its size?
How stable could the described relationship be?

Thank you for your interest and answers.


texts: At what distance wouldn't the Sun cast a visible shadow anymore?

texts: How far from the Sun would a (dwarf) planet have to be so that the Sun is dim enough that it doesn't cast a shadow when you're standing on the planet? What magnitude is the border in brightness beyond which a shadow isn't visible anymore?


texts: Is it accurate to compare comets to clouds and rain?

texts:
I'm trying to avoid an opinion-based question, so before I outline the comparison I'm proposing, I will qualify the specific facts that yield this comparison. By focusing answers on the relative accuracy of those facts, hopefully we can avoid primarily opinion-based answers.

Facts

Over the course of millions of years, planetary bodies, especially those closer to the sun, tend to lose their water and atmosphere as it is blown away by solar wind, and radiated or boiled off by other forces, unless they have a strong magnetosphere to repel the bulk of that solar wind, or enough gravity to hang on to their matter. On a much faster time scale, this also happens to comets as they approach the sun; thus, the tail.
Over the course of millions of years, bombardment of planets by comets can increase the water and gas on a planet, or at least counter-balance the gradual loss of the same (depending on other factors like proximity to the sun, size, and magnetic protection).
The Oort cloud is composed mostly of icy bodies (including solids that would be a gas at Earth temperatures). And it is from this region that comets "fall" toward the inner solar system.

Also Fact?
So I started wondering what ultimately happens to the water and gas that gets blown off of planetary bodies. I would guess that given the outward pressure of solar wind and solar radiation, the direction of such material is generally outward, away from the sun. Well, where to? Media coverage of the departure of Voyager 1 from our solar system into the realm of interstellar space alerted me to the idea that there is an outer edge to solar influence, and I would guess that this is where the lighter molecules, like water and gases would tend to land. And indeed there is a collection of similar material out there at that solar radius.

Given these facts, is it accurate to view the Oort cloud as behaving similarly to a terrestrial cloudscape? Our clouds form at the altitude where water tends to deposit after it evaporates from the heated surface of the Earth. It gathers there and coalesces to a form which is then overcome by the opposing force of gravity. Then it returns from whence it came. Similarly, I imagine water and gases ultimately being driven from the heat of the sun to an “altitude above its surface”, where ultimately, over a much longer time scale, that material coalesces until other forces eventually drive them back toward the hot sun. Is this an accurate understanding? Is this comparison justified?


texts: Can a planet's gravity rip out its moon's atmosphere?

texts:
We all know that solar flares can rip a planet's atmosphere if the magnetic field is weak.Solar flares travel at tremendous speed so when they hit the gas molecules of a atmosphere the molecules are ejected from the gravitational pull and thrown away.

Can a giant planet's gravity pull out its moons atmosphere?Is gravity enough to pull its atmosphere or do we need an instantaneous force/impact to blow away an atmosphere?


texts: Why is metallicity important in the death of stars?

texts:
I always thought that mass was the sole determinant of a star's fate. Then I saw the table here.
So why does metallicity influence a star's ability to become a black hole or neutron star? Does it have as much influence as mass?


texts: Total luminosity of total population of galaxies!

texts:
I'm trying to get the total luminosity of a total population of galaxies.
I have the Schechter Function like

$$\Phi(L) = \frac{\Phi^{*}}{L^{*}} \left(\frac{L}{L^{*}}\right)^{\alpha} \exp \left(\frac{L}{L^{*}}\right)$$

and I need to the total luminosity:

$$L_{tot} = \int_0^\infty L\Phi(L)dL = \Phi_{0}L^{*}\Gamma(2+\alpha)$$

but when I substitute the Schecter functions I don't get the $\Gamma$ function form. Could you have any tips for that?

And I need to show that the galaxies that more contributed to the luminosity are those have $L^{*}$

Thank you in advance.


texts: How can I calculate how far through the day the prime meridian is of different planets

texts:
I'm trying to create a widget that shows the 'time' on different planets. It will show how far through the day/night cycle (as a percentage) a point on the planet is. It has been easy to scale down the cycle from 24 hr to eg ~10hr day of Jupiter however I am struggling to work out how I can 'set' the time.

For Mars this has been straight forward, using the comparison of MTC and UTC of when the Curiosity rover landed but I can't find similar data for other planets to anchor time from earth to the other planets.

Is there a data base for this kind of information?


texts: Is it plausible to use other stars for the proposed FOCAL mission instead of the Sun?

texts:
For some time, the far-reaching and speculative idea of using the Sun as a gravitational lens has been floating around. See this and this. This would require sending a spacecraft about ~550 AU of a distance away from Earth, so the idea is not currently realistic.

But I am wondering, is there any plausibility to using other stars (like the Alpha Centauri stars)?

Is there there enough parallax for this to be useful? How far away must a satellite go to be able to manipulate parallax to its advantage? Are there any shortcomings to this idea that make the FOCAL proposition superior? And of course, are there any shortcomings to both propositions?

I can imagine there's not much freedom in where the so-called lens can be used, but maybe the limited field of view can still be useful if the satellite could roam the solar system?

I'm curious if anyone can give useful feedback or calculations.


texts:
Let us say that we have an object so its total velocity is defined as

\begin{equation} v_{tot} = v_{pec} + V_{rec} \end{equation}

Where

\begin{equation} V_{rec} = H_0r \end{equation}

and \begin{equation} V(z) = \frac{cz}{1+z}[1+\frac{1}{2}(1-q_0)z - \frac{1}{6}(1-q_0-3q_0^2+j_0)z^2] ~~(4)\end{equation}

for small z.

So my first question is what is the $z$ value here? Is it the observed redshift or the cosmological redshift?

Also, the relationship between observed and cosmological redshift is given.

\begin{equation} 1+z_{obs} = (1 + z_{cos})(1 + z_{earth})((1 + z_{sun})(1 + z_{source})(1 + z_{gravity})    \end{equation}

If we are using the cosmological redshift then by using above equation we can write,

\begin{equation} z_{cos} = \frac{1 + z_{obs}} {(1 + z_{earth})((1 + z_{sun})(1 + z_{source})(1 + z_{gravity})}-1    \end{equation}

So is this what we put in (4)?

Edit: For the source you can look here https://arxiv.org/abs/1907.12639 Eqn(16) and (18)


texts: Elemental abundances in the crust of exoplanets and accessibility to them

texts: Suppose I am orbiting a rocky planet (density = 4.4 g/cm3) two-thirds the mass of Mars in the Habitable Zone of Gliese 251. This planet has a thin atmosphere, presumably from out-gassing, still has internal heating, has a weak magnetic field, but has no sign of tectonic activity. I need to mine precious and radioactive elements to replenish my ship. Gliese 251 only has about 8% of the metallicity of the sun. Can I assume that the planet (theoretically) would have about 8% of the elements (per volume of crustal material) as the Earth? Would lack of tectonic activity make them much harder to get to?


texts: Where can I find source data for the graph of metric expansion over time, as shown in this ESA web page?

texts:
This web page from ESA/Hubble, shows the size of the universe over time. (I know that "size of the universe" is a gross simplification and open to multiple definitions and ambiguity, but this kind of graph is roughly what I'm after).

But no source data is cited. This is the closest I can find to what I'm after - where can I find source data to match this graph?


texts: Why is the opening in the Anglo-Australian Telescope's dome so small?

texts:
Many older or "classic" telescope domes have a horizon-to-zenith opening in the dome, and this helps speed up the thermal equilibration between the inside and outside air, decreasing turbulence and its effects upon resolution.

Source

Source

But the Anglo-Australian Telescope has second mechanism that tightly constrains the vertical extent of the opening as well, leaving just a tiny hole barely big enough for the telescope to see the sky.

What are the benefits of each of these, and how do the minimal aperture domes deal with temperature differences?

below: Screen shot from the video A 2dF night at the Anglo-Australian Telescope found here.


texts: What is the standard reference point for measuring speed?

texts:
Speed, as we know, doesn't exist without first having a reference point. We then say that the reference point isn't moving at all, and speed is then measured in relation to the reference point.

What is the standard reference point when determining the speed of objects in astronomy? Earth?


texts: Will Gaia detect inactive neutron stars?

texts:
Will the astrometric precision of the Gaia space telescope be able to detect the gravitational influence of cold old solitary neutron stars on the movements of stars? At least in a statistical sense to estimate how common they are around here, which could say something about how large the population III stars were.

And if an inactive neutron star could be precisely located thanks to its gravitational influence, would it be possible to observe it directly?


texts: How long time since a typical asteroid collided with another asteroid?

texts: How pristine and ancient are asteroids really? I suppose they melt and reform when they collide with each other. How frequently do they collide with each other?


texts:
In this answer they say:

  While the conversion of mass matter† to energy in the Sun's core now represents a loss of mass proper matter, it turns out that that energy (trapped in the Sun and slowly diffusing towards the surface) will have the same gravitational attraction as the matter it came from until it actually escapes the Sun!

Now I expect that when mass is converted to energy, it has become an electromagnetic wave that can propagate through matter or empty space.

My question is: How does energy (from fusion reactions still inside the sun) still have gravitational attraction?


texts: Constructing a periscope/telescope - trouble with lenses

texts:
Not sure if this is the best place for this, maybe physics would be a better fit, but I'm trying to build a periscope and am running into an issue.

Traditional mirror periscopes work well, but when I look into the one end of the periscope, I see the reflected image at the end of a long tube. I figure that using some combination of lenses, you should be able to make it look as if the position of your eye/head was actually at the top of the tube looking out at the world rather than looking through a tube.

My thinking here is that when you look through binoculars or a telescope, you don't see the inside of the tube, just the enlarged image. So shouldn't you be able to do the same thing but without the magnification?

I purchased some lenses from Amazon with focal lengths of 200mm and -100mm and based on some optics simulators assumed(incorrectly apparently) that I could arrange them 10cm apart and then another set reversed on the other side like so:

`````()````````````````)(```````````)(``````````````()
200mm````````-100mm``-100mm```````200mm

() = convex lens
)( = concave lens

The thinking here was that one set would condense the image down so that the walls of the tube were no longer visible within that viewing angle, and then the other reversed set would reverse that and the image would be back at normal size to my eye, it would just look as though my eye were actually at the opposite end of the tube.

This didn't work. The image is close to life size, but the tube looks even longer: a foot long tube has turned into a 3 foot long tube, and the circle I can see the world through is very small.

Is this a pipe dream? I feel like it must be possible if they used periscope in military applications for years before electronics and fiber-optic cables were invented.

Where did I go wrong? Any guidance would be hugely appreciated.

Edit: some follow up questions

Am I stuck having a small field of view out of the end? Or would using a lower focal point(wide angle) lens allow me to see a wider angle?

Also, is there any way to have a variable length tube? For instance if I'm looking around a wall, is there a way to install the lenses so that the tube could be "telescoped" out to look around a thicker wall?


texts: The age of the universe is 13.798±0.037 billion years, yet the age of HD 140283 is 14.46±0.8 billion years, how this can be the case?